Relative humidity measuring apparatus



y 1953 E. s. RITTNER ETAL RELATIVE HUMIDITY MEASURING APPARATUS 3 Sheets-Sheet 1 Filed Feb. 8. 1949 FIG. 7

INVENTORS EDMUND S. RITTNER 8 SAMUEL%FINE BY GENT FIG.

May 19, 1953 E. s. RITTNEI E TAL 2,638,783

RELATIVE HUMIDITY MEASURING APPARATUS Filefi Feb. 8, 1949 3 Sheets-Sheet 2 AMP.

T '1 1 mm X X SEN ITIVE SERVO I I E .1 I I I J FIG. l2

INVENTORS EDMUND S. RITTNER y 1953 E. s. RITTNER ETAL RELATIVE HUMIDITY MEASLRING APP AT S 3 Sheets-Sheet 3 Filed Feb. 8. 1949 FIG. H

I I I SERVO l I\PHIASE x SENSITIVE FIG. I3

INVENTORS EDMUND S. RITTNER SAMUEL FINE BY ENT AMF? CONTROL UNIT OSCIL- LATOR -40 REFRIGERATION BATH 46 R 31 $0 BRIDGE vih'e'rein 1,1 is the den I t'h'e *absoliite -tempmture. The independent parameters =aiitlf z'jo "can a'l'so 'Be t'eniperature dependent 'aitnduh in men eases the tempera- "ture dependence iii to is srii'all relative at that of "tiohs of the above "t ie for a a'Bsig-ried tempera-tin 1 variable in the etb've with menivehtmn m I H -'v'ice embed-vine atemperatu'e responsive-ele- In Equation "2, a 'ig would exhibitat eribgtenee is tit-be- Patented May 19, 1953 2,638,783 H U v etentive HUMIDI'TY MEA'SURIEG APPARATUS nl un js, ti e, Peekskill, and sa mueiinme, Xgrlx, N.J assignorsjto P ilips Debora-- 'tories, Inc., Irvington-on-Hudson, N. Y. Y

Ourihv'ention relates to relative humidity mea er-mg a paratus.

-In numerous chemical and physical pro blenrs,

it is"often necessaiytoevaluateexponential'funo- "tions involving temperature for-example, expon'entfal function's er *the form hdeiit variable "and fl is Exponential fuiicticins offtlie'above type arise the determination 'oi the -a-liquitl'or solid, of the pm-tial p'ressure (if; olve'n-tvapdr a'b 'ove solu- =tion, or the e'iiiiilib 1m "bonfstaht or "a chemical *re'actio'nfof the sfidific ea'ction rate velocity of chemical 'ractionfidf 'the absolute humidity of "air, and or the relativejlfi'uinidity'of air, and in related physical 'or "chemical ijroblern's.

It isOft'e'n des'ired'to obta ih solutionsof equw r'g'e number of n"'which*case it is time- 'c'onsumiii'g and memoirs to perfdirn the necessary operations in the solution-or the-e111 tion by conventionalmathematical thermos. This is especia'll-y the-case when the parameter It varies with temperature; v

It is therefore ano-bject (if our invention to provide a- "simple outing 'deviii which is adapted to yield-pleas described aided-on at an assignedvanes-tr e V I attained-inacferdame ofa L mputing denien't when eenautah'ceerienges accordance with the law:

the value a; ethquet U the conducn T the O].

which fediioes' e'ipbnent to ad'iirihsiohiess number. j

Efein'ent's: having a; behavior conforming v to Equation "2' be ruined of -eubttanc's irelies 'of the dependent:

3 Claims. (01; vs-e38) means.

uring-rliitive'hiimidity m a'doraehce with Equawater} reference as "had- 2 v. longing to the 'gerie'ralfcl'asjs "disarm-conductors and insulators, such "as, tna'mumsumae, 'zino .oxid e, titaniilnl dioitide, cobaltoxide containing'a lithium impur t nickel"ofiiiiecoritaining a'lithiui'n impurity "dubious; oxi'de',- ndv'ai' ioiis inipiirity type 's'emi cbhciiictors' described in Belgian Patent No. "475,570. a rurtnr jfeaturepr the inventicin, elements constituted bf'uh semi-conductors "and 'i'n'slil'atofs 'al' rei ned-into netwerks, whereb the veilue ofafid tempe ature dependence er the quivalent conduction activation-eh- 'fg'y is matched-to the value and temperature dependence bf'the 'bafalfiter A 111 "Equation "1.

law; i a n Figure 10 shows one form of computing device in accordance with t l'ieirivention for directly determining relative humidity of the atmosphere; and .I 7

Figure 11 shows another form of computing device in accordance withthe invention for directly determining relative humidity of the atr'nosphere;

Figures IQA and 11A ehow details; "rspef" ti'v'ly of the Figures r0 and ii wet bulb measuring The invention will be described with particular reference to the determination of both relative the absolute hurhiditiesof However, it is to be understood that the invention is applicable to other physical or chemical problems and the following description is inten'ded to be illustrativephiy; i v

n erdej f'tfi at 1 which after making simplifying assumptions and '3 integrating, can be expressed in the following form:

P being the equilibrium pressure, Po being the equivalent value of the equilibrium pressure of Water at an infinitely high temperature, Q the latent heat of evaporation of water, R the universal gas constant having a value 1.9864 calories per mole per degree centigrade and T the absolute temperature in degrees Kelvin.

The absolute humidity in air is defined as the concentration or partial pressure of water-vapor actually present in the air. Referring to Fig. l, the curve shows the equilibrium vapor pressure of ice and liquid water as a function of temperature over a range of temperatures extending from below the freezing point to approximately the boiling point of water. This curve also represents substantially the partial pressure of water vapor in air in equilibrium above ice and liquid water as a function of temperature over the same temperature range.

Let the ordinate P1 represent the partial pressure of water vapor (and correspondingly the absolute humidity) in the atmosphere at temperature T2. The equilibrium vapor pressure corresponding to this temperature is then given by the ordinate P2. P.1, however, has substantially the same value as the equilibrium pressure of water-vapor at a lower temperature T1, commonly referred to as the dew-point temperature. P1, can be calculated by using Equation 3 as follows:

. In accordance with the invention the equilibrium partial pressure P1 at the dew-point temperature is determined. by measuring the conductance of a semi-conductor having an activation energy equivalent toQ. Thus from Equation 1:

01:0 RTl when U is specified in calories per mole, and hence, P1 is directly proportional to o'.

The relative humidity H is defined as the ratio of the partial pressure of water in air to the saturation pressure at the same temperature:

It is to be noted that the relative humidity H is simply the ratio of the conductances of the two semi-conductive elements, one maintained at ambient temperature and the other maintained at the dew-point temperature.

A simple apparatus illustrating one method for measuring relative humidity based on Equation 8 is shown in Figs. 12 and 13. The two semiconductive elements S01, 802 are connected in opposite arms of a simple bridge circuit (Fig. 12) and constitute resistances R1 and R4, respectively. The other elements of the bridge are constituted by a variable resistanceRz and a fixed resistance B3. A galvanometer G is connected between the junctions of R1 and R4, and R2 and R3. The other two junctions are connected to a suitable source of potential. One of the semi-conductor elements SC; is maintained at room temperature and the other SC 2(R4) is maintained at the dewpoint temperature.

Fig. 13 shows one arrangement for maintaining R4 at the dew-point temperature. Referring to Fig. 13, the semi-conductive element R4 is in good thermal contact with but insulated electrically from a block of metal "40 having a mirror surface 4|. An induction heating coil 42, arranged to be energized by an R. F. oscillator 43, surrounds the upper portion of the block 40. A photoelectric cell 44 is arranged to collect that portion of the light emitted by source 39 which is reflected from the mirror surface 4| of the block 40 and the output of the cell 44 is coupled to a control unit 45, which in turn is coupled to the R. F. oscillator 43. The block of metal 40 is cooled by a refrigeration bath 46. The .apparatus is operated by cooling the block 40 and the mirror surface 4! by the bath 46 until a deposit of moisture appears on the mirror surface 4|. The light reflected from the mirror surface 4| is decreased in intensity due to this depositzof moisture resulting in a changed output from the photocell which actuates the control unit 45, which in turn increases the power output of the oscillator 43 until the heat lost by conduction to the refrigeration bath 46 through the block 40 just balances the heat input from the oscillator 43. At the balance point, there is no increase or decrease in the thickness of the moisture deposit and the temperature of the block 40 is then at the dew-point temperature. Since R4 is in good thermal contact with the block 40, it too is also at the dewpoint temperature. ,Variable resistor R2 is then adjusted until the bridge ,(Fig. 12) balances at which time the galvanometer reads zero, and the relative humidity H is then determined as the ratio of R2 to R3. It is obvious that for convenience, a dial can be attached to-Rz for reading H directly. It is further obvious that the setting of R2 can be automatically controlled by replacing galvanometer G by a suitably chosen resistor and by feeding the voltage drop generated across said resistance by flow of unbalance current into a conventional amplifier, the output of which in turn is fed to a simple servomechanism mechanically coupled to the arm of resistor R2, the unbalance current causing the servomechanism to adjust R2 until the bridge balances. Furthermore, the foregoing arrangement can be employed in recording or controlling, as well as inaesavss solvent vapor over a solution, specific reaction rate velocities. and the like, the conductance or resistance of elements constituted by semi-conductive substances or insulators is determined as illustrated in Figures 4 to 9a. In Fig. 4, the

- conductance u of the element SC which is constituted by a semi-conductive substance or an insulator is conveniently measured by measuring the potential drop with a voltmeter V across a fixed resistor'R having a resistance'very much smaller than the reciprocal of a.

In Fig. 5, either the conductance or resistance of an element R4 constituted by a semi-conductivesubstance or an insulator SC can be measured with a conventional bridge circuit having resistance elements R1, R2 and R3. To measure the resistance of R4, resistor R3 is made variable and the bridge balanced, since R4 is directly proportional to R3. To measure the conductance of R4, R2 is made variable and the bridge balanced, since the conductance l/R4 is directly proportional to R2.

In Fig. 6, the element SC constituted by a semiconductive substance or an insulator is connected in series with a source of A.-C. potential and acapacitor C of much lower impedance than the reciprocal of so that the potential measured across the capacitor is directly proportional. to

the conductance 0' of SC. This circuit is also useful for controlling auxiliary equipment since the time-constant and hence the natural damping frequency of this circuit will be governed by the value of conductance of SC.

Fig. '7 shows a simple circuit for measuring the resistance X of an element SC constituted by a semi-:conductive substance or an insulator by connecting it in series with a battery and an ammeter A calibrated to read in units of resistance.-

Fig. 8 shows a simple circuit for measuring the resistance X of an element SC constituted by a semi-conductive substance or an insulator by measuring the potential drop thereacross when the SC is connected in series with a constant current barretter B and a battery.

Fig. 9 shows a simple circuit for measuring the resistance: of an element SC constituted by a semi-conductive substance or an insulator by means of a series inductance L of much lower impedance than the reciprocal of the con-, ductance. In this case, the potential developed across-the inductance with alternating current flowing through the series combination is directly proportional to the value of the resistance of the semi-conductor.

Fig. 9a shows one variant of a simple resonant circuit in which the element SC constituted by a semi-conductive substance or insulator controls the damping of the series combination of in ductance L and capacitor C to thereby control auxiliary circuits.

In addition to the dew-point method previously described, psychrometric methods are advantageously employed in measurements of relative humidity. In order to employ the semi-conductors hereinbefore described as elements of a psychrometer for making rapid and automatic determinations of relative humidity, the elements according to the invention are preferably employed in special computing networks such as those illustrated in Figs. 10 and 11 and about to be described. I

It can be shown, both theoretically and empirically that the relative humidity of air can be expressed as follows:

' the equilibrium water-vapor pressure at the drybulb temperature, P1 is the equilibrium watervapor pressure at the wet bulb temperature, B is the barometric pressure and C is a constant term.

The wet and dry bulb temperatures are easily attained as is well known to those skilled in the art. Quantities proportional to the equilibrium vapor pressures are obtained by means of the elements according to the invention as described hereinbefore. This information plus the barometric pressure is supplied to an electrical computing network about to be described and the relative humidity is automatically indicated and/or recorded. Moreover by means of servomechanisms not illustrated, auxiliary equipment, for example air-conditioning apparatus, can be controlled to maintain a precise desired value of relative humidity.

The electrical computing network about to be described is the analogue of Equation 10 rearranged as follows:

Referring to Fig. 10, the computer is connected through a transformer, generally designated I, having a plurality of secondary windings 2, 3, 4 and 5 and 3t. Commencing with secondary winding 2, an element 6, constituted by a semiconductive substance or substances, is maintained at a dry-bulb temperature and connected in series with a fixed resistance 1, the output of which is supplied to an amplifier 8. The output of the latter is applied to a potentiometer 9, thence through a resistance I I] to an amplifier II, a phase-sensitive servo-amplifier I2 and a motor I3 which is arranged to drive the movable arm I4 of the potentiometer 9. Secondary winding 3 is connected in series with a second element I 5, constituted by a semi-conductive substance or substances, maintained at the wet-bulb temperature and a fixed resistance I6, the output of which is supplied through resistor I! to amplifier I I.

Secondary winding 4 is connected in series with a barretter I8, and a temperature sensitive resistance IQ for measuring the dry bulb temperature; the value of the resistance of I9 is directly proportional to the absolute temperature. The

.output of It is coupled to an amplifier 20. The

output of the latter is coupled through potentiometer 2|, the position of which is proportional to the barometric pressure, and resistor 22 to amplifier I I.

Secondary winding 5 is connected in series with a barretter 23, and a temperature sensitive resistance 24, the output of which is proportional to the absolute temperature, for measuring the wet-bulb temperature. The output of resistance 24 is coupled to an amplifier 25, the output of the latter being coupled through potentiometer 26, whichis mechanically coupled to potentiometer 2|, and resistor 21 to amplifier I l.

The setting of potentiometers 2| and 26 can be accomplished either manually, or automatically by a pressure sensitive electric indicator and a servo-mechanism.

As will readily be seen, the input to amplifier II in terms of voltages corresponds to the sum of the terms on the left of Equation 11. If this input is not equal to zero, a voltage is applied to servo I! which compares this voltage with a reference voltage obtained from secondary winding 36 on the transformer and senses the phase of the voltage supplied by amplifier ll. Servo l2 operates motor l3 to move arm l4 until the input to amplifier H is zero. The final setting of arm I4 is a direct measure of the relative humidity and can also be coupled through other servo-mechanisms to auxiliary equipment. Fig. 10A shows the well-known means for maintaining the semi-conductor element [5 and the resistor 24 at the wet-bulb temperature by simply inserting both the element l 5 and the resistor 24 in a wick 31 having one end immersed in a pool of water 38 and conducting a stream of air thereover.

Fig. 11 shows one possible variation of the device shown in Fig. 10. In this figure, the reference numerals which are the same as those in Fig. indicate identical or similar components. In this circuit arrangement, however, thermocouples 30 and 3| are employed to obtain the difference in temperature between the dry-bulb and the wet-bulb temperatures. The output of the thermocouples is applied to an amplifier 34. The output of the latter is coupled through potentiometer 32, provided with an adjustable arm for adjusting the potentiometer to include a calibration for barometric pressure, and resistor 33 to amplifier ll. Fig. 11A shows a device similar to that shown in Fig. 10A for maintaining the thermocouple 3| and the semi-conductor 15 at the wet-bulb temperature by inserting them in a wet wick 39 supplied by a pool of water 39' and conducting a stream of air thereover.

Obviously, these circuits are capable of many modifications without affecting their basic character or in affecting the basic. principle of the invention which has been just described.

While numerous examples of the substances employed have been given, numerous applications have been indicated, and numerous manners in and circuit arrangements have been described for applying the invention into practice, it is of course recognized that the number of modifications are legion and it would be impossible to describe all of these modifications. It is therefore our desire that the invention described herein be construed as liberally as possible in view of the prior art.

What we claim is:

1. A device for measuring relative humidity in air comprising first and second semi-conductors each having a conductance r conforming to the relationship where do is the conductance of the semi-conductor at an infinite temperature, U is the activation energy of the semi-conductor and has a value equivalent to the latent heat of evaporation of water at a given temperature, It is a constant having the same units as U per degree of temperature, and T is the absolute temperature,

means to maintain the first semi-conductor at the ambient temperature of air, means to maintain the second semi-conductor at the dew-point temperature of air, and means to measure the ratio of the conductances of the two semi-conductors to thereby obtain a quantitative determination of the relative humidity of the air.

2. A device for measuring relative humidity in air comprising first and second semi-conductors each having a conductance a conforming to the relationship on being the conductance of the semi-conductor at an infinite temperature, U being the activation energy of the semi-conductor and having a value equivalent to the latent heat of evaporation of water at a given temperature, k being constant with the same units as U per degree of temperature, and T being the absolute temperature, means to maintain the first semi-conductor at the ambient temperature, means to maintain the second semi-conductor at a temperature corresponding to a wet-bulb, means to measure the ambient and wet-bulb temperatures, and calculating means coupled to said semi-conductors and said temperature measuring means capable of solving the equation the values of P1 and P2 corresponding to the values of conductance of the first and second semi-conductors, respectively, T1 and T2 being the ambient and wet-bulb temperatures, respectively, B being the barometric pressure and C a constant factor.

3. A device for measuring the relative humidity in air as claimed in claim 2 in which the calculating means is an analogue computing network capable of solving the equation the values of P1 and P2 corresponding to the values of conductance of the first and second semi-conductors, respectively, T1 and T2 being the ambient and wet-bulb temperatures respectively, B being the barometric pressure and C a constant factor.

EDMUND S. RITTNER.

SAMUEL FINE.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,956,386 Gruss Apr. 24, 1934 2,016,660 Weeks Oct. 8, 1935 2,098,650 Stein Nov. 9, 1937 2,349,860 Hainer May 30, 1944 FOREIGN PATENTS '5 Number Country Date 558,299 Great Britain Dec. 30, 1943 

